This invention relates generally to method and apparatus for the logging of earth boreholes and particularly to method and apparatus for providing an indication of the earth formation porosity which is substantially independent of the salinity of such formations.
It is well known in the prior art to utilize a pulsed neutron source and a pair of spaced radiation detectors to provide an indication of the formation porosity. With such prior art devices, the near-spaced detector is gated on during some interval between the neutron bursts and the far-spaced detector is similarly gated on during the same gating interval between bursts. The signals from the first detector are then combined with the signals from the second detector to result in a ratio which is indicative of the formation porosity.
For example, in U.S. Pat. No. 3,379,882 to Arthur H. Youmans which issued on Apr. 23, 1968, and which is assigned to the assignee of the present application, it is disclosed with respect to FIG. 13 of that patent that the ratio is achieved by combining signals from the two detectors in a ratio circuit and specifically that the gating intervals should be identical.
Furthermore, in U.S. Pat. No. 3,621,255 to Robert J. Schwartz which issued on Nov. 16, 1971, there is disclosed a concept much like that of the aforementioned Youmans patent which is said to provide an indication of the formation porosity by producing signals from each of the detectors which are combined in ratio circuitry to provide a signal which is related to the porosity of the formation. The circuitry described in the Schwartz patent speaks of automatically adjusting the gates in the two detectors in accordance with the formation characteristics being logged but does not particularly indicate how the gates associated with one detector will relate in time to the gates associated with the other detector. FIG. 2 of that patent and its related disclosure indicate that the gates do occur some time along the curve segments 54 and 55 but otherwise is silent as to their relationship to each other.
It has been discovered, however, that when using identical gate intervals with the two detectors to provide a formation porosity ratio, the resultant ratio is highly dependent upon formation salinity.
It is therefore the primary object of the present invention to provide new and improved method and apparatus for indicating formation porosity which is substantially independent of formation salinity.
The objects of the invention are accomplished, generally, by well logging method and apparatus which utilize one gating interval for the first detector and a different gating interval for the second detector. By providing a ratio of the signals indicative of detected radiation occurring during the two gating intervals, a log is provided of the formation porosity which is substantially independent of the formation salinity. In a more specific embodiment, the short-spaced detector gating interval, while being shorter in duration than the long-spaced gating interval, does have its beginning point in coincidence with the beginning point of the long-spaced detector gating interval.